Heat sink assembly for cooling electronic modules

ABSTRACT

An array of electronic modules on the top of a circuit board is cooled by a heat sink secured to the bottom of the board. Vias transfer heat through the board to a compliant interface pad which permits heat to flow to the heat sink while maintaining electrical isolation between the vias. The heat sink is held on the board by push pin assemblies including springs that compress the interface pad between the circuit board and the heat sink. The body of each push pin assembly includes flexing legs supporting locking barbs, and flex of the legs is limited by a strut defining a closed end slot in the shank of the body.

FIELD OF THE INVENTION

The present invention relates to improvements in heat sink assembliesfor cooling electronic modules mounted on a circuit board.

DESCRIPTION OF THE PRIOR ART

Electronic modules include numerous circuits operating at high speed andgenerate substantial heat. In many applications, it is necessary to coolthe modules in order to maintain specified temperatures and assure thatthe modules function properly and reliably. In the past, variousapproaches have been used to cool electronic modules. A common approachis to mount a finned metal heat sink in heat transfer relation upon thetop of the module. U.S. Pat. Nos. 5,285,347 and 5,353,863 discloseexamples of arrangements wherein a finned heat sink is mounted upon thetop of an electronic module.

With emerging module types such as ball grid array (BGA) modules, theleads may be relatively fragile. One disadvantage of the knownarrangement wherein a heat sink is secured to the top of the module isthat the mass of the heat sink is added to the mass of the module andmust be supported by the module leads soldered to the circuit board. Theleads may lack sufficient strength to support both the module and a heatsink.

With electronic modules such as memory chips, an array of many discretechips may be mounted to the surface of a single circuit board. Each ofthe chips of the array must be cooled. Using a discrete heat sink foreach chip as disclosed in U.S. Pat. No. 5,285,347 results in anexpensive assembly having many parts and assembly operations. Inaddition, this conventional assembly permits temperature variationsamong the different modules, and this can result in timing performancevariations.

Some of the disadvantages of using a separate heat sink for each chip anbe avoided by employing a larger heat sink used simultaneously to cool anumber of chips. U.S. Pat. Nos. 5,285,350 and 5,370,178 disclose coolingmodules or heat sinks that overlie an array of several electronicmodules in order to cool them all simultaneously. The heat sinksdisclosed in these patents are secured by fasteners or clips to thecircuit board and are supported above the chips in heat transferrelationship to the top surfaces of the chips. This mounting arrangementrequires space above the top surface of the circuit board and above thetop surfaces of the chips sufficient for accommodating the heat sink andpermitting a sufficient flow of air over the heat sink. In someapplications it would be desirable to cool modules without the necessityfor additional space above the circuit board.

In some applications it is desirable to have access to the chips mountedon a circuit board. For example in an array of memory chips, it can benecessary to rework the assembly by removing and replacing a defectivechip of the array. This is typically done by heating the assembly toreflow the solder holding the leads to the board. The presence of one ormore heat sinks can interfere with rework of the assembly.

To facilitate rework, various approaches have been used to mount a heatsink so that it can be removed. Flexible clips are used in the assemblyof U.S. Pat. No. 3,353,863. Threaded screws are used in the assembly ofU.S. Pat. No. 5,370,178.

U.S. Pat. No. 5,384,940 discloses a pin with an open-ended, bifurcatedpoint that can be inserted through a heat sink and into a hole in acircuit board in order to secure the heat sink above a surface mountpackage. Prongs on the point engage the board when the pin is fullyinserted, and a spring urges the heat sink down into contact with thepackage. A similar product sold by Aavid Engineering, Inc. of Laconia,N.H. is made of a nylon plastic material. The heat sink mounting pindisclosed in U.S. Pat. No. 5,384,940 and sold is subject todisadvantages. The sharp points defined by the open-ended bifurcationcan catch on the board surface and interfere with movement of the pinthrough the board. The free standing prongs at the point of the pin aresubject to excessive deformation and are lacking in strength. The prongscan take a set if not fully inserted and later fail to spring backresulting in an inability to secure the heat sink to the board.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an improvedheat sink assembly for cooling of electronic modules. More specifically,other objects of the invention are to provide a heat sink assembly thatminimizes parts and labor assembly costs, that does not require addedspace above the circuit board on which the modules are mounted, thatdoes not apply stress to component leads and does not interfere withaccess to components for rework and that encourages uniform moduletemperatures. Another object is to provide a module cooling heat sinkassembly that overcomes disadvantages of those used in the past.

It is also an object of the invention to provide an improved push pinassembly for mounting a heat sink to a circuit board. Related objectsare to provide a pin that is inserted easily into a hole withoutcatching on the edge of the hole; to provide a pin having increasedstrength and resistance to deformation; and to provide a heat sinkmounting pin that does not become permanently deformed if improperlyinstalled.

In brief, in accordance with the present invention, there is provided anassembly for cooling a plurality of electrical modules mounted on thetop surface of a circuit board. The circuit board includes an array ofvias extending between the top and bottom surfaces of the circuit board.Each electrical module includes a plurality of leads, each lead beingattached to a conductive region on the top surface of the circuit boardconnected to one of the vias. The assembly of this invention includes ametal heat sink aligned with the array of vias. A plurality of mountingelements attaches the heat sink to the bottom of the circuit board. Athermally conductive element contacts the bottom surface of the circuitboard and the heat sink for conducting heat from the vias to the heatsink.

In brief, in accordance with another aspect of the invention there isprovided a push pin assembly for mounting a heat sink to a circuitboard. The heat sink and the circuit board have aligned mounting holes.The push pin assembly includes a pin body formed of a molded plasticmaterial and having a push head at a first end and a point at the secondend for insertion through the mounting hole of the heat sink and throughthe aligned mounting hole of the circuit board from an entry surface toan exit surface of the circuit board. The body has a shank extendingbetween the head and the point. The second end of the body has barbsprojecting radially beyond the shank for engaging the exit surface ofthe circuit board after insertion. A coil spring is captured around theshank and beneath the head for urging the heat sink toward the entrysurface of the circuit board. A slot extends diametrically through thepoint and the shank to define a pair of spaced apart legs and to impartflexibility to the legs. A strut portion of the body closes the end ofthe slot and interconnects the ends of the legs at the distal end of thepoint.

BRIEF DESCRIPTION OF THE DRAWING

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiment of the invention illustrated inthe drawings, wherein:

FIG. 1 is a side elevational view of a circuit board and electronicmodule assembly including a heat sink assembly embodying the presentinvention;

FIG. 2 is an exploded isometric view of the components of the circuitboard and electronic module assembly of FIG. 1;

FIG. 3 is a greatly enlarged fragmentary vertical sectional view of partof the circuit board and electronic module assembly of FIG. 1;

FIG. 4 is an enlarged side elevational view of one of the heat sinkmounting push pins of the assembly of FIG. 1; and

FIG. 5 is an exploded isometric view of the push pin and its associatedspring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having reference now to the drawings, in FIG. 1 there is illustrated acircuit board and electronic module assembly generally designated as 10including an module cooling assembly generally designated as 12embodying the principles of the present invention. In general theassembly 10 includes a circuit board 14 and a plurality of modules 16mounted upon the top surface 18 of the board 14. In the illustratedembodiment there is an array of four modules 16, however the principlesof the invention can be employed with arrays of more or fewer modules.

Elements of the assembly 10 are illustrated in more detail in FIG. 3.The circuit board 14 is formed of an electrically insulating materialsuch as fiberglass and includes a plurality of vias 20 each extendingfrom the top surface 18 to a bottom surface 22 of the board. The vias 20are tubular, cylindrical segments of conductive metal, normally copper.A lower end 24 of each via is exposed at the bottom surface 22 of theboard. At the top surface 18, an upper end 26 of each via is exposed andis in electrical contact with a conductive copper region or contact pad28. The circuit board 14 includes other conductive regions and layersincluding internal layers selectively contacting the vias 20, but theseare not related to the present invention and are omitted from thedrawing for clarity. The term "upper" or "top" of the circuit board 16means the side of the circuit board upon which components are mountedregardless of eventual physical board orientation and the term "lower"or "bottom" means the side opposite the component mounting side.

The modules 16 may, for example, be static random access memory (SRAM)chips in the form of plastic ball grid array or PBGA modules havingnumerous electrical leads 30. One example of a BGA module has 119 leadsin a seven by seventeen rectangular array with a lead to lead spacing of0.050 inch, although the invention can be used with many otherconfigurations and module types. The vias 20 and contact pads 28 arearrayed to correspond to the array of leads 30 and there is one via 20and contact pad 28 for each lead 30. The leads 30 are in the form ofballs of solder. The modules 16 are mounted to the top surface of thecircuit board 16 by melting or reflowing the solder of the leads 30 toform an electrical connection and mechanical attachment between eachlead 30 and its corresponding contact pad 28. Conductive layers orregions of the circuit board 14 selectively contact the vias 20 or thepads 28 and provide paths for signal, ground and power connections tothe modules 16.

Heat is generated by the operation of the electrical elements within themodules 16. In the illustrated embodiment of the invention, theelectronic module cooling assembly 12 removes heat from the modules tomaintain the temperatures of the modules within a specified operatingrange to assure consistent and reliable module performance and life.

In general, the cooling assembly 12 includes a heat sink 32, mountingelements in the form of push pin assemblies 34 for attaching the heatsink 28 to the circuit board 16, and a thermally conductive element 36for conducting heat to the heat sink 28. The heat sink 32 is attached tothe lower surface 22 beneath the circuit board 16. As a result, thecooling assembly does not require space above the circuit board 16 andthis is an advantage in applications where space above the board islimited. In addition, the assemblies 34 attach the heat sink 32 directlyto the board and the mechanical attachment of the modules 16 to thecontact pads 28 is not required to support the mass of the heat sink 32in addition to the mass of the module 16.

The heat sink 32 is made of a thermally conductive metal such asaluminum and includes a base 38 and numerous fins 40 to provide a largesurface area for the dissipation of heat. Preferably the fins 40 areoriented parallel to one another and parallel to the flow of cooling airwithin an enclosure. Each corner of the heat sink base 38 is providedwith a mounting hole 42, and the circuit board 16 is provided withmounting holes 44 in the same pattern. To mount the heat sink 32 ontothe circuit board 14, the holes 42 are aligned with the board holes 44and the push pin assemblies 34 are inserted in an upward directionthrough the aligned holes 42 and 44. The push pin assemblies 34resiliently bias the heat sink 32 toward the bottom surface 22 of thecircuit board 14.

The circuit board vias 20 are used as paths for the flow of heat fromthe modules 16 to the heat sink 32. The thermally conductive element 36conducts heat from the lower end 24 of each via 20 to the base 38 of theheat sink 32. In the illustrated embodiment the thermally conductiveelement is a planar, conformable, soft, elastic, electrically insulatingand thermally conductive interface pad sandwiched between the bottomsurface of the circuit board and the heat sink. A suitable material isT-FLEX 240 (200 SERIES) Thermally Conductive Conformable Gap Filler soldby Thermagon, Inc., 3256 West 25th Street, Cleveland, Ohio 44109-1668.Information about this material may be found at the web sitewww.thermagon.com.

FIGS. 4 and 5 illustrate one of the push pin assemblies 34. It includesa pin body 46 formed of a molded plastic material and a spring 48. Thebody 46 is a unitary, one-piece, homogeneous molded part having a pushhead 50 at one end and a point 52 at the opposite end. A shank 54extends axially between the head 50 and point 52. A pair of barbs 56extend radially from the body 46 in the region of the point 52. Eachbarb 56 has a sloped entry surface 58 and a radial lock shoulder 60.

The spring 48 is a coil spring that is installed over the barbs 56 andaround the shank 54. The spring closely encircles the shank 54 and iscaptured between the lock shoulder 60 and an inner shoulder 62 of thehead 50. A neck 64 at the transition of the head 50 and the shank 54locates the spring 48 concentrically on the body 46.

A slot 66 extends diametrically through the shank 54 and point 52 anddefines a pair of parallel, axially extending legs 68 and 70 spacedapart by the slot 66. The slot 66 extends perpendicular to a diameterline between the barbs 56, and one of the barbs 56 is located on eachleg 68 and 70. Because of the characteristics of the molded plasticmaterial of the body 46 and because of the slot 64, the legs areflexible and permit the barbs 56 to resiliently flex radially inwardly.

A strut portion 72 joins the ends of the legs 68 and 70 and defines theend of the slot 66. The strut 72 prevents relative radial movement ofthe ends of the legs 68 and 70 and stiffens the legs while permittingdeflection of the portions of the barbs adjacent to the radial lockshoulders 60. As a result, the slot 66 is closed rather than open ended,and each leg flexes in the nature of a bridge fixed at both ends ratherthan as a cantilever beam free at one end.

The strut portion 72 is circular in shape and defines a blunt, circularend of the point 52. The circumference of the circular strut merges withthe leading ends of the sloped barb surfaces 58. This configurationfacilitates insertion of the point 52 into a mounting hole. There are nosharp ends and no free flexing leg ends to catch on a surfacesurrounding a mounting hole.

To install the cooling assembly 12 onto the circuit board 14, theinterface pad 36 is located between the base 38 of the heat sink 32 andthe bottom surface 22 of the circuit board. The interface pad is alignedwith and overlies all of the vias 20 extending from the modules 16. Themounting holes 42 of the heat sink 32 are aligned with the mountingholes 44 of the circuit board 14, and the push pin assemblies 34 areinstalled.

The head 50 of the push pin assembly 34 is held in the hand and thepoint 52 is inserted through the aligned holes 42 and 44. The bluntpoint 52 defined by the strut 72 fits easily into the holes 42 and 44without catching or sticking. The barbs in their relaxed state areseparated by a distance greater than the mounting hole diameter. Thesloping barb surfaces 58 guide the pin body 46 axially through the holesand react with the holes to cause inward radial deformation of the legs68 and 70, permitting the barbs 56 to pass through the mounting holes.When the pin assembly 34 is fully inserted, legs 68 and 70 flex radiallyoutwardly to their relaxed positions and the radial lock shoulders 60 ofthe barbs 56 overly the top surface 18 of the circuit board 18 toprevent withdrawal of the pin assembly 34. As seen in FIG. 1, the point52 extends above the circuit board 14.

If the push pin assembly 34 is inserted only part way through a mountinghole, the barbs 56 can remain flexed radially inwardly for a long periodof time. In the illustrated embodiment of the invention the body 46 ismolded of polycarbonate rather than a material such a nylon. Thisminimizes the tendency of the legs 68 and 70 to take a set orpermanently deform with the barbs retracted. The tendency to permanentlydeform is also opposed by the strut 72. If the assembly 34 is fullyinserted after remaining partly inserted, the legs 68 and 70 are able toflex radially outwardly to lock the body 46 in place.

The diameter of the coil spring 48 is larger than the diameter of themounting hole 42. The spring abuts the base 38 of the heat sink 32 asthe pin assembly 34 is installed. When the pin assembly 34 is fullyseated, the spring 34 is held in compression between the inner shoulder62 of the head 50 and the base 38 of the heat sink 32. The forcesapplied by the springs 48 urge the heat sink 32 toward the bottomsurface of the circuit board 14 and compress the soft, compliantinterface pad 36 between the heat sink 32 and the circuit board 14. As aresult, the interface pad is pressed into intimate contact with thelower end 24 of each of the array of vias 20 and is pressed intointimate contact with the heat sink base 38.

As the modules 16 operate, heat generated within the modules flows fromthe modules through the leads 30 and contact pads 28 to the vias 20.Heat then flows from the vias 20 and into the interface pad 36, whilethe electrical isolation of the vias 20 is maintained by theelectrically insulating interface pad 36. Heat flows from the interfacepad 36 to the heat sink 32 and is dissipated into the surrounding airfrom the fins 40. Because all of the modules 16 are in common heattransfer relation with the single heat sink 32, temperature variationsbetween individual modules are reduced and the electrical timing of themodules is maintained consistent with improved timing performance. Thecooling assembly 12 can easily be removed and does not interfere withrework of the array of modules 16.

While the present invention has been described with reference to thedetails of the embodiment of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

What is claimed is:
 1. An assembly for cooling a plurality of electricalmodules mounted on the top surface of a circuit board, the circuit boardincluding an array of vias extending between the top and bottom surfacesof the circuit board, each via including a tubular, cylindrical segmentof thermally conductive material, each module including a plurality ofleads, each lead being attached to a conductive region on the topsurface of the circuit board connected to one of the vias; the assemblycomprising:a metal heat sink aligned with the array of vias; a pluralityof mounting elements removeably attaching said heat sink to the bottomof the circuit board; and a thermally conductive element contacting thebottom surface of the circuit board and said heat sink for conductingheat from said vias to said heat sink; said thermally conductive elementcomprising a discrete, conformable, elastic, electrically insulating andthermally conductive interface pad having opposed flat planar surfaces,said pad being removeably sandwiched in elastic compression between thebottom surface of the circuit board and the heat sink, said conductiveelement being in direct contact with both the bottom ends of said viasat the bottom surface of the board and with said heat sink.
 2. Theassembly of claim 1, said mounting elements comprising push pinsextending through aligned holes in said heat sink and in the board. 3.The assembly of claim 2, said push pins each including a body formed ofa molded plastic material and having a push head at a first end, a pointat the second end for insertion through the mounting hole of the heatsink and through the aligned mounting hole of the circuit board from thebottom to the top of the circuit board, and a shank between said pushhead and said point.
 4. The assembly of claim 3, said second end havingbarbs projecting radially beyond said shank for engaging the top surfaceof the circuit board after insertion.
 5. The assembly of claim 4 furthercomprising a coil spring captured around said shank and beneath saidhead for urging the heat sink toward the bottom surface of the circuitboard.
 6. The assembly of claim 5 further comprising a slot extendingdiametrically through said point and said shank to define a pair ofspaced apart legs and to impart flexibility to said legs.
 7. An assemblyfor cooling a plurality of electrical modules mounted on the top surfaceof a circuit board, the circuit board including an array of viasextending between the top and bottom surfaces of the circuit board, eachmodule including a plurality of leads, each lead being attached to aconductive region on the top surface of the circuit board connected toone of the vias; the assembly comprising:a metal heat sink aligned withthe array of vias; a plurality of mounting elements attaching said heatsink to the bottom of the circuit board; and a thermally conductiveelement contacting the bottom surface of the circuit board and said heatsink for conducting heat from said vias to said heat sink; said mountingelements comprising push pins extending through aligned holes in saidheat sink and in the board; said push pins each including a body formedof a molded plastic material and having a push head at a first end, apoint at the second end for insertion through the mounting hole of theheat sink and through the aligned mounting hole of the circuit boardfrom the bottom to the top of the circuit board, and a shank betweensaid push head and said point; said second end having barbs projectingradially beyond said shank for engaging the top surface of the circuitboard after insertion; a coil spring captured around said shank andbeneath said head for urging the heat sink toward the bottom surface ofthe circuit board; a slot extending diametrically through said point andsaid shank to define a pair of spaced apart legs and to impartflexibility to said legs; and a strut portion of said bodyinterconnecting the ends of said legs at the distal end of said point.8. The assembly of claim 1, said modules comprising ball grid arraymodules and said leads comprising balls of solder.